1. Field of the Invention
The present invention relates to a novel trailing edge shape of a laminar-flow airfoil, in which a head-lowering pitching moment around an aerodynamic center, which is a problem peculiar to a laminar-flow airfoil, is suppressed.
2. Description of the Related Art
A boundary layer on a surface of a main wing of an airplane is a laminar-flow boundary layer at a leading edge, but changes from the laminar-flow boundary layer to a turbulent-flow boundary layer toward a trailing edge. A friction drag on the surface of the main wing is smaller at the laminar-flow boundary layer than at the turbulent-flow boundary layer. For this reason, in order to decrease the drag on the main wing, it is desirable that a transition point at which the laminar-flow boundary layer changes to the turbulent-flow boundary flow is displaced toward the trailing edge, to thereby extend the region of the laminar-flow boundary layer as much as possible.
A laminar-flow airfoil of “6-series” developed by NACA in early 1940s can suppress the drag better than the conventional laminar-flow airfoil. However, when a portion of a wing surface in the vicinity of a leading edge is rough, the largest lift disadvantageously tends to decrease largely, leading to a great problem during takeoff or landing of the airplane.
Thereafter, NASA developed NLF(1)-0215F and NLF(1)-0414F in 1977 and 1983, respectively. These laminar-flow airfoils enables a reduction in the drag, but have a problem of causing a large head-lowering pitching moment. Moreover, because these laminar-flow airfoils are for use in a low-speed range, they have a problem of causing drag-divergence phenomenon at an early stage, of a subsonic speed range.
In HSNLF (1)-0213 developed by NASA in 1984 for use in a high subsonic speed range, a drag-divergence phenomenon is difficult to generate, and a head-lowering pitching moment is small. However, the largest lift in a lower Reynolds number range is small and the capacity of an inner-wing fuel tank is insufficient because the wing thickness is about 13% of a wing chord length, leading to a difficulty in ensuring mileage.
U.S. Pat. No. 4,858,852 and U.S. Pat. No. 5,318,249 disclose known airfoils having a feature in the shape of a trailing edge.
In the airfoil disclosed in U.S. Pat. No. 4,858,852, a trailing edge of the airfoil for use in a transonic speed range is of a blunt shape having a thickness, and the distance between an upper wing surface and a lower wing surface diverge in the vicinity of the trailing edge so that the wing thickness is increasing toward the trailing edge, thereby providing an increase in lift and a decrease in drag.
In the airfoil described in U.S. Pat. No. 5,318,249, a portion of each of an upper surface and a lower surface of the airfoil for use in a transonic speed range is largely curved downwardly in the vicinity of a trailing edge, thereby preventing laminar-flow separation at an upper wing surface in the transonic speed range.
Generally in a laminar-flow airfoil in which a laminar-flow boundary layer region extends along towards a trailing edge, a negative pressure on an upper wing surface is also large at the trailing edge, so that the trailing edge side lift accounts for a large proportion in the entire wing lift. As a result, a head-lowering pitching moment about an aerodynamic center is disadvantageously increased. In order to countervail the head-lowering pitching moment, it is necessary to increase a negative lift generated by a horizontal empennage. If the negative lift generated by the horizontal empennage is increased, the lift on the entire airplane is decreased and the drag on the horizontal empennage is increased, whereby the drag on the entire airplane is disadvantageously increased. It is also necessary to increase the area of the horizontal empennage and to increase the moment arm extending from a gravity center position to the horizontal empennage, disadvantageously leading to further increases in the weight and drag.